In deepwater drilling rigs, marine risers extending from a wellhead fixed on the ocean floor have been used to circulate drilling fluid or mud back to a structure or rig. The riser must be large enough in internal diameter to accommodate a drill string or well string that includes the largest bit and drill pipe that will be used in drilling a borehole. During the drilling process drilling fluid or mud fills the riser and wellbore.
After drilling operations, when preparing the wellbore and riser for production, it is desirable to remove the drilling fluid or drilling mud. Removal of drilling fluid or drilling mud is typically done through a displacement using a completion fluid.
Because of its relatively high cost, this drilling fluid or drilling mud is typically recovered for use in another drilling operation. Displacing the drilling fluid or drilling mud in multiple sections is desirable because the amount of drilling fluid or mud to be removed during completion is typically greater than the storage space available at the drilling rig for either completion fluid and/or drilling fluid or drilling mud.
It is contemplated that the term drill string or well string as used herein includes a completion string and/or displacement string. It is believed that rotating the drill string or well string (e.g., completion string) during the displacement process helps to better remove the drilling fluid or mud along with down hole contaminants such as mud, debris, and/or other items. It is believed that reciprocating the drill or well string during the displacement process also helps to loosen and/or remove unwanted downhole items by creating a plunging effect. Reciprocation can also allow scrapers, brushes, and/or well patrollers to better clean desired portions of the walls of the well bore and casing, such as where perforations will be made for later production.
During displacement there is a need to allow the drilling fluid or mud to be displaced in two or more sections. During displacement there is a need to prevent intermixing of the drilling fluid or mud with displacement fluid. During displacement there is a need to allow the drill or well string to rotate while the drilling fluid or mud is separated into two or more sections.
During displacement there is a need to allow the drill string or well string to reciprocate longitudinally while the drilling fluid or mud is separated into two or more sections.